What is a Supermassive Black Hole?
Imagine the mass of a whole galaxy with its billions of stars smushed together to fit inside the Earth's much smaller orbit around the Sun. A supermassive black hole is an object that heavy and that small.
The gravity of supermassive black holes sucks in huge amounts of gas and dust and is so strong that nothing—not even light—can escape if it gets too close. Supermassive black holes are found in the center of galaxies, including our own. Just one is millions of time bigger and heavier than a regular black hole, which is usually just one collapsed star.
If light cannot escape a black hole, then why do they look so bright?
Astronomers cannot see supermassive black holes directly, but they can see the gas and dust around them. This material orbits around the black hole very fast and heats up to billions of degrees. The black hole itself is invisible because light cannot escape it. But the gas and dust near it shines brighter than the rest of the galaxy just before getting sucked in.